Transducers of the type with which the present application is concerned are utilized for the conversion of acoustic energy into or from another form of energy, usually electrical energy, and depend upon the vibration of a mechanical element of relatively high acoustic impedance being converted into or generated from said other form of energy. In a practical system, to or from which acoustic energy is to be transmitted or received, this medium typically being air which has a very low acoustic impedance. The nature of this coupling determines the efficiency of the system, its frequency response, and the directionality of the propagation of the energy in the medium.
One widely used form of acoustic transducer assembly utilizes an axially deformable cylindrical element such as a piezoelectric crystal held in an open end of a cylindrical support such as a tube. Sound waves emanate from the end, or radiating aperture, of the tube when the outer end surface of the element vibrates in response to an excitation of the element as by electrical stimulation. Such a transducer assembly is commonly utilized for transmission and/or reception of sound in a gaseous medium, the sound usually being of a high frequency such that the sound wavelength in the medium is smaller than the dimensions of the radiating aperture.
The radiation pattern of sound emitted from such a transducer approximates that of a plane circular piston operating within an infinite baffle. It is well known that the directivity of such a transducer is a function of the ratio of the diameter of the radiator to the sound wavelength in the propagating medium, so that a radiator of larger diameter will exhibit a higher degree of directivity than will one of smaller diameter while propagating waves of the same length into the same medium. Thus, for a given directivity, a lower sound frequency requires a larger transducer element.
Of particular interest is the acoustic power and bandwidth of sound radiated from a transducer such as that described above. A major and well known problem exists in the transmission of sound between a gaseous medium of low acoustic impedance and a high impedance acoustic transducer assembly such as that abovementioned. The problem is present irrespectively of whether the sound is radiated from the transducer assembly into the medium or from the medium into the transducer assembly, and is manifested by a substantially reduced coupling and bandwidth of the acoustic energy transferred between the source and the medium. In the case of a piezoelectric crystal and an air environment the difference in impedance is enormous, being of the order of 10,000 to one or greater.
The essence of the coupling problem is that the low impedance gaseous environment offers very little opposition to the motion of the high impedance piezoelectric crystal so that little work is done by the crystal in imparting motion to the gaseous environment.
A well known means whereby the crystal may be made to do more work on, and thereby impart more energy into a gaseous medium is to arrange that the crystal be stimulated at one of its natural resonant frequencies thereby causing the motion of the crystal surfaces to be greater by a factor of ten or twenty or more times. In such manner, the same crystal surface area works against the same opposition offered by the gaseous environment but through a much greater distance each time the crystal surface moves through one cycle of its motion. More work is therefore done and more energy is imparted to the gaseous environment for each cycle of motion of the crystal surface. Even then, comparatively little power is transferred to the medium, and since there is little damping of the crystal oscillation, the bandwidth is very narrow and the resultant ringing effect makes it impossible to transmit and receive sharply defined pulses of sound energy with very short attack and decay times.
Another well known means whereby the crystal may be made to do more work on, and thereby impart more energy to a gaseous medium is to place an intermediate structure such as a rigid cone or diaphragm, whose frontal dimension is greater than that of the crystal, between the crystal and the gaseous environment. Such an arrangement suitably constructed according to well known principles results in a greater area (according to the ratio of the frontal area of the cone or diaphragm to that of the crystal) of the gaseous environment being displaced by the motion of the crystal. Accordingly, a larger area moving through the same distance against the same opposition offered by the gaseous environment results in more work being done by the crystal than would be the case if the crystal were operated without benefit of the intermediate structure. Depending upon the mass and rigidity of the cone or diaphragm, the performance of the device can be influenced in various ways, but if a highly directional output is required, only a modest improvement in output can be achieved, since the size of the diaphragm is limited by the necessity for maintaining a coherent wavefront and a gross impedance mismatch remains.
A third well known means whereby the vibrating element may be made to do more work on, and thereby impart more energy into a gaseous medium is to place one or more impedance transforming transmission line sections between the crystal and the gaseous environment. This latter method of impedance matching has been fully described in U.S. Pat. No. 3,674,945 issued July 4, 1972 to Hands for "Acoustic Matching System". The operation of this latter method depends upon the acoustical properties of the matching section or sections which are placed between the high impedance crystal and the low impedance gaseous medium and upon those of the crystal and the gaseous environment themselves. The effect of a properly devised matching structure of this type is that it allows the motion at the interface between the structure and the gaseous environment to be much greater than the motion at its opposite end at the interface between the structure and the crystal surface. Thus a short powerful stroke at the high impedance end of the structure, at the crystal face, is transformed into a much longer but less forceful stroke at the low impedance end of the structure at the interface with the gaseous medium.
The severity of the impedance mismatch between a piezoelectric crystal and a medium such as air is readily demonstrated. For example, in the case of a piezoelectric crystal being utilized without a matching structure for transmission of sound power into air, the crystal may have to be driven at such large amplitudes of pulsation that the crystal may fracture, while with the insertion of some form of matching structure between the crystal and the air environment, the same sound power can be transmitted into the air by driving the crystal at substantially reduced amplitudes of pulsation which do not induce crystal fracture.
It should be noted that while the aforementioned use of a structure, constituting sections of acoustical transmission line so chosen as to effect an impedance match between the high impedance crystal and a low impedance air environment, does provide improvement in sound transmission as compared to the absence of any such matching structure, nevertheless, the efficiency of the arrangement remains extremely low, and the degree of coupling to the medium is not high enough to provide any significant damping of the oscillation of the crystal which must therefore be damped by other means if a widened bandwidth is required.
Proposals have been made to match the impedance of a high impedance driving source such as a piezoelectric crystal to a lower impedance environment such as air by the use of an intermediate structure embodying a vibrating plate or disc, but it has not been possible heretofore to achieve such a match without sacrificing directionality and/or bandwidth.
An example of such a proposal is provided by Scarpa U.S. Pat. No. 3,891,869 issued June 24, 1975, wherein there is disclosed an acoustic transducer assembly including a driving element comprising a piezoelectric generator in the form of a disc with a high mass backing element bonded to one face and an acoustic wave transformer bonded to the other. The wave transformer element varies in cross-section in an axial direction, comprising discs of maximum dimension at the generator face and at the radiating face. Beginning at line 48, column 2 of the disclosure the statement is made that the transformer, including the disc, functions to step down the impedance by increasing the area of contact at the radiating surface, which moves in small arc vibrations at high velocity.
In the device described in the Scarpa patent, a highly directional field of sound emission is not a requirement. In point of fact, a main feature of the device is that phase differences across the vibrating disc cause the central lobe of radiation to be suppressed, and cause the side lobes to be enhanced to the point that a major portion of the energy radiated is radiated away at an angle of about 45 degrees to the main axis of the device.
Another prior art proposal is described in a paper by J. A. Gallego-Juarez, G. Rodriguez-Corral and L. Gaete--Garreton, published in the November, 1978 issue of Ultrasonics.
In that paper there is described a transducer utilizing a stepped vibrating plate to effect an impedance match between a source of ultrasound vibrations and a gaseous environment, whilst providing highly directional radiation. Although an effective impedance match is obtained by the device its bandwidth is extremely narrow, typically being about 10 hertz for a device operating at about 20,000 hertz corresponding to a Q of about 2,000. A device with such a high Q is suitable for production of continuous sound at a fixed frequency, but is not suitable for use in pulsed echo-ranging applications where it is necessary that the transducer exhibit a much lower Q providing a bandwidth of at least 5 to 10 percent of the resonant frequency.
Hitherto, transducer systems suitable for pulsed echo-ranging applications in gaseous mediums have been of the type disclosed in U.S. Pat. No. 3,674,945, or more simple and inefficient coupling methods have been used, together with some mechanical and/or electric means for damping the vibrating element thus leading to very low efficiencies. A further problem with such systems arises in applications where a substantial range is required. Since absorption of sound energy by gaseous media increases with frequency, longer ranges require not only greater power but lower frequencies, and this means that to obtain the required directionality and power output, larger transducer elements must be used. The piezoelectric materials widely used for such elements are both expensive and massive, and whilst it would be entirely possible to produce a transducer system in according with U.S. Pat. No. 3,674,945 which will perform satisfactorily at 10 kHz, the mass and cost of such a system would be excessive for normal commercial applications.
According to the invention a broadly tuned directional acoustic transducer system comprises a plate having a radiating surface and a higher flexural mode resonance at substantially the operating frequency of the system, and a transducer element of much smaller effective area than the radiating surface of the plate and connected thereto for excitation or response to said higher flexural mode resonance, wherein at least alternate antinodal zones of the radiating surface of the plate are coupled to a gaseous propagation medium by means formed to low-loss acoustic propagation material of much lower acoustic impedance than the plate and applied at least to said alternate antinodal zones of the radiating surface thereof in a thickness selected to differentiate at least one of the relative phase and the relative amplitude of the radiation from adjacent antinodal zones sufficiently to reduce substantially mutual cancellation, in the far field and in the desired direction of radiation, of sound radiated into said medium from adjacent antinodal zones of the plate. Preferably the plate is axisymmetrically resonant and in presently preferred forms of the invention a disc shaped plate is used coupled axially to the transducer element, the axis of the plate and the disc also being the directional axis of the system. With a disc shaped plate, the covering material is arranged in concentric rings covering adjacent antinodal zones, the thickness of adjacent rings being different so as to produce coherency of radiation in the axial far field. In one embodiment of the invention the thickness of material covering alternate zones is zero, i.e. alternate zones are uncovered. The matching into the propagation medium from the covered zones can thus either be made so much better than that from the uncovered zones that substantially no phase cancellation occurs in the axial far field, or sufficient phase shift can be introduced in sound radiated from the covered zones to substantially reduce cancellation. Alternatively the whole radiating surface of the plate may be covered by material, of thickness such that there is both phase shift of radiation from alternate zones, and acoustic impedance matching between the plate and the propagation medium, usually air. The covering material need not be uniform, and adjacent zones could be covered by different material, or the material could comprise layers of different materials or have graded properties provided that the desired phase and/or amplitude modification is achieved. The improved coupling of the system to the medium damps the system thus reducing its Q and rendering it capable of use in echo-ranging techniques without external damping.